Method for making a lithium-sulfur battery separator

ABSTRACT

A method for making a lithium-sulfur battery separator includes providing a separator substrate comprising a first surface and a second surface opposite to the first surface; and forming a functional layer on at least one of the first surface and the second surface. A method of forming the functional layer includes providing a carbon nanotube layer comprising a plurality of carbon nanotubes; etching the carbon nanotube layer to form defects on surfaces of the plurality of carbon nanotubes; and forming a hafnium oxide layer on the defects to form a carbon nanotube/hafnium oxide composite layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims all benefits accruing under 35 U.S.C. § 119 fromChina Patent Application No. 201710415506.2, filed on Jun. 5, 2017, inthe China Intellectual Property Office, the contents of which are herebyincorporated by reference. The application is also related to copendingapplications entitled, “LITHIUM-SULFUR BATTERY SEPARATOR ANDLITHIUM-SULFUR BATTERY USING THE SAME”, filed * * * (Atty. Docket No.US69749).

FIELD

The present disclosure relates to a method for making lithium-sulfurbattery separator.

BACKGROUND

In a lithium-sulfur battery, the cathode is made of sulfur and the anodeis made of elemental lithium. During electrical discharge process, theelemental lithium loses electrons to become lithium-ion, and the sulfurreacts with the lithium-ion and electrons to produce lithium sulfides. Areaction equation is: S₈+16Li⁺+16e⁻¹=8Li₂S. A lithium-sulfur battery hasadvantages of low-cost, environmental friendliness, good safety, andhigh theoretical specific capacity.

A separator is an important component in the lithium-sulfur battery. Theseparator separates the cathode and the anode to avoid an internalshort-circuit. However, the lithium-sulfur battery separator obtained bya conventional method is difficult to inhibit polysulfide diffusion. Thepolysulfide would be shuttled between the cathode and the anode, anirreversible damage to a structure of the cathode containing sulfurwould be occurred. Thus the specific capacity and cycling stability ofthe lithium-sulfur battery would be limited.

BRIEF DESCRIPTION OF THE DRAWINGS

Implementations of the present technology will now be described, by wayof example only, with reference to the attached figures, wherein:

FIG. 1 is a structure schematic view of one exemplary embodiment of alithium-sulfur battery separator.

FIG. 2 shows surface morphology of a functional layer of thelithium-sulfur battery separator in FIG. 1.

FIG. 3 is a structure schematic view of one exemplary embodiment of alithium-sulfur battery separator.

FIG. 4 is a structure schematic view of one exemplary embodiment of alithium-sulfur battery separator.

FIG. 5 is a structure schematic view of one exemplary embodiment of alithium-sulfur battery separator.

FIG. 6 shows cross-section morphology of a functional layer of thelithium-sulfur battery separator in FIG. 1.

FIG. 7 is a structure schematic view of a functional layer of thelithium-sulfur battery separator in FIG. 1.

FIG. 8 is a flow chart of one exemplary embodiment of a method formaking a lithium-sulfur battery separator.

FIG. 9 is a structure schematic view of one exemplary embodiment of alithium-sulfur battery.

FIG. 10 shows constant current charge-discharge test curves of alithium-sulfur battery of Example 1 and a lithium-sulfur battery ofComparative Example 1.

FIG. 11 shows charge-discharge voltage profiles at different currentdensities of the lithium-sulfur battery of Example 1.

FIG. 12 shows charge-discharge voltage profiles at different currentdensities of the lithium-sulfur battery of Comparative Example 1.

FIG. 13 shows cyclic stability at different charge/discharge rates ofthe lithium-sulfur battery of Example 1 and Comparative Example 1.

FIG. 14 shows a cyclic stability of the lithium-sulfur battery ofExample 1 and the lithium-sulfur battery of Comparative Example 1 at 0.2C.

FIG. 15 shows self-discharge test curves of the lithium-sulfur batteryof Example 1 and the lithium-sulfur battery of Comparative Example 1after standing for 20 days.

DETAILED DESCRIPTION

The disclosure is illustrated by way of example and not by way oflimitation in the figures of the accompanying drawings in which likereferences indicate similar elements. It should be noted that referencesto “another,” “an,” or “one” embodiment in this disclosure are notnecessarily to the same embodiment, and such references mean “at leastone.”

It will be appreciated that for simplicity and clarity of illustration,where appropriate, reference numerals have been repeated among thedifferent figures to indicate corresponding or analogous elements. Inaddition, numerous specific details are set forth in order to provide athorough understanding of the embodiments described herein. However, itwill be understood by those of ordinary skill in the art that theembodiments described herein can be practiced without these specificdetails. In other instances, methods, procedures, and components havenot been described in detail so as not to obscure the related relevantfeature being described. Also, the description is not to be consideredas limiting the scope of the embodiments described herein. The drawingsare not necessarily to scale, and the proportions of certain parts havebeen exaggerated to illustrate details and features of the presentdisclosure better.

Several definitions that apply throughout this disclosure will now bepresented.

The term “substantially” is defined to be essentially conforming to theparticular dimension, shape, or other feature which is described, suchthat the component need not be exactly or strictly conforming to such afeature. The term “comprise,” when utilized, means “include, but notnecessarily limited to”; it specifically indicates open-ended inclusionor membership in the so-described combination, group, series, and thelike.

Referring to FIG. 1, one embodiment is described in relation to alithium-sulfur battery separator 10. The lithium-sulfur batteryseparator 10 comprises a separator substrate 110 and at least onefunctional layer 120. The separator substrate 110 comprises a firstsurface and a second surface opposite to the first surface. The at leastone functional layer 120 is coated on at least one surface of the firstsurface and the second surface. Referring to FIG. 2, in one embodiment,the lithium-sulfur battery separator 10 comprises at least twofunctional layers 120, and the at least two functional layers 120 arecross-stacked with each other.

The separator substrate 110 can be a film. In an example, the separatorsubstrate 110 can be a microporous polyolefin membrane. The microporouspolyolefin membrane comprises a polypropylene (PP) film, a polyethylene(PE) film, or a multilayer composite film of the PP film and the PEfilm. The separator substrate 110 comprises a plurality of micropores.In one embodiment, the separator substrate 110 is a PP film having athickness of 20 micrometers.

The number of the functional layers 120 can be selected according toactual needs. Referring to FIG. 3, in one embodiment, both the firstsurface and the second surface are coated with the functional layer 120,the number of the functional layers 120 on each surface of the firstsurface and the second surface is about 5 layers to about 15 layers.Referring to FIG. 4, in one embodiment, the functional layer 120 is onlycoated on the first surface, and the number of the functional layers 120is from about 5 layers to about 15 layers. In another embodiment, thefunctional layer 120 is only coated on the first surface, and the numberof the functional layers 120 is 10 layers.

Referring to FIG. 5, an electron microscope image of the functionallayer 120 shows that the functional layer 120 is a smooth porousstructure comprising a plurality of pores. A pore size of each of theplurality of pores is from about 1 micrometer to about 3 micrometers.

A thickness of the functional layer 120 is from about 0.1 micrometer toabout 0.3 micrometers. FIG. 6 shows a cross-sectional topography of10-layer functional layers 120 stacked with each other, it can be seenthat a total thickness of 10-layer functional layers 120 is about 1.5micrometers.

Referring to FIG. 7, the functional layer 120 comprises a carbonnanotube layer 122 and a hafnium oxide (HfO₂) layer 124. The carbonnanotube layer 122 comprises a plurality of carbon nanotubes. TheHfO₂layer 124 comprises a plurality of HfO₂ nanoparticles. The HfO₂nanoparticles are adsorbed on surfaces of the carbon nanotubes. Thethickness of HfO₂ nanolayer is about 1 nanometer to about 5 nanometers.In one embodiment, the thickness of HfO₂ nanolayer is about 3nanometers. In one embodiment, a surface of each carbon nanotubecomprises uniform and continuous defects, the HfO₂ nanoparticles beingadsorbed on the uniform and continuous defects. The HfO₂ nanoparticlesform a continuous HfO₂ layer on the surfaces of the carbon nanotubes.The uniform and continuous defects are depressions on the surfaces ofthe carbon nanotubes. Each depression can be a dot-shaped depression ora linear depression. The uniform and continuous defects can be formed byetching the surfaces of the carbon nanotubes. Each uniform andcontinuous defect can further comprise a functional group, thefunctional group is beneficial for a surface adsorption of Hf source andan adsorption of polar polysulfides. The functional group can alsoimprove electrolyte wettability of the functional layer. In oneembodiment, the functional group is an oxygen-containing functionalgroup.

The carbon nanotube layer 122 can be a porous structure. The carbonnanotube layer 122 can be one carbon nanotube film or at least twocarbon nanotube films stacked and crossed with each other. In oneembodiment, the carbon nanotube layer 122 comprises at least two drawncarbon nanotube films stacked and crossed with each other. In anotherembodiment, the carbon nanotube layer 122 consists of at least two drawncarbon nanotube films stacked and crossed with each other. A largenumber of carbon nanotubes in the drawn carbon nanotube film can beoriented along a preferred direction, meaning that a large number of thecarbon nanotubes in the drawn carbon nanotube film are arrangedsubstantially along the same direction. A minority of carbon nanotubesin the drawn carbon nanotube film may be randomly aligned. However, thenumber of randomly aligned carbon nanotubes is very small and does notaffect the overall alignment of the majority of carbon nanotubes in thedrawn carbon nanotube film.

When the number of the carbon nanotube films of the carbon nanotubelayer 122 is small, the lithium-sulfur battery separator does notprovide good mechanical support for a volume exchange of sulfur and aconductive path will be short, which is adverse for polysulfideadsorption and conversion. When the number of the carbon nanotube filmsof the carbon nanotube layer 122 is large, it is difficult to ensurethat the surface of each carbon nanotube adsorbs HfO₂ nanoparticles, andas a result, the shuttle effect of polysulfides generated during thereaction is difficult to limit. In one embodiment, the carbon nanotubelayer 122 comprises 2-4 layers of drawn carbon nanotube films stackedand crossed with each other, an angle between adjacent carbon nanotubefilms is about 90 degrees. In one embodiment, the carbon nanotube layer122 comprises two drawn carbon nanotube films stacked and crossed witheach other, an angle between the two carbon nanotube films is about 90degrees.

When a thickness of the HfO₂ layer 124 is small, the shuttle ofpolysulfide generated during the reaction cannot be effectively limited.When the thickness of the HfO₂ layer 124 is large, insulation of theHfO₂ layer 124 will result in low electrode reaction kinetics, whichwill lead to poor electrochemical performance. In one embodiment, thethickness of the HfO₂ layer 124 is from about 1.0 nanometer to about 5.0nanometers. In one embodiment, the thickness of the HfO₂ layer 124 isfrom about 2.5 nanometers to about 3.5 nanometers. In anotherembodiment, the thickness of the HfO₂ layer 124 is about 3.0 nanometers.

An area density of the functional layer 120 is about 0.08 mg/cm² toabout 0.10 mg/cm². The area density of the functional layer 120 isdefined as mass per one square centimeter of the functional layer 120.In one embodiment, the area density of the HfO₂ layer 124 is about 0.087mg/cm².

The carbon nanotubes of the carbon nanotube layer 122 can form adispersed carbon nanotube conductive network. The HfO₂ layer 124 and thedispersed carbon nanotube conductive network can greatly improve thesurface interaction between the functional layer 120 and solvents. When1.5 μL of deionized water is dropped onto the functional layer 120, acontact angle between the functional layer 120 and the deionized wateris about 13.4°, which shows that the functional layer 120 has excellentwettability. Such excellent wettability of the functional layer 120greatly increases the active sites for electrochemical reaction betweenthe active materials and electrolyte. Moreover, the highly polararizedpolyslufides can be effectively adsorbed and utilized by the functionallayer 120.

The lithium-sulfur battery separator 10 has many advantages. First, thecarbon nanotube layer 122 has a large specific surface area, whichallows uniform deposition of the HfO₂ nanoparticles on the carbonnanotube layer 122. A charge transfer process for the surface adsorptionand conversion of polysulfides is accelerated. Second, a surfacecatalytic adsorption capability on the polysulfide of the HfO₂ layer 124is excellent; a contact area between the polysulfide and thelithium-sulfur battery separator is increased by a well-dispersedconductive carbon nanotube network and the ultra-thin HfO₂ layer.Thereby, the surface adsorption reaction of the lithium sulfur batteryseparator is greatly improved, and the polysulfide shuttling phenomenonis greatly suppressed.

Referring to FIG. 8, a method for making the lithium-sulfur batteryseparator 10 is disclosed. The method comprises any of the followingsteps:

-   -   step (S1), providing a separator substrate 110, wherein the        separator substrate 110 comprises a first surface and a second        surface opposite to the first surface; and    -   step (S2), forming a functional layer 120 on at least one        surface of the first surface and the second surface, which        comprises sub-steps of:        -   step (S21), providing a carbon nanotube layer 122 comprising            a plurality of carbon nanotubes on at least one surface of            the first surface and the second surface;        -   step (S22), forming a plurality of uniform and continuous            defects on surfaces of the plurality of carbon nanotubes by            etching the carbon nanotube layer 122; and        -   step (S23), forming a hafnium oxide (HfO₂) layer 124 on            surfaces of the plurality of carbon nanotubes to form a            carbon nanotube/HfO₂ composite layer.

In more detail, in step (S2), the functional layer 120 can be directlylaid on the separator substrate 110, and then the functional layer 120is infiltrated with ethanol to combine the functional layer 120 with theseparator substrate 110.

In one embodiment, forming at least two functional layers 120 on atleast one of the first surface and the second surface, the at least twofunctional layers 120 are stacked with each other.

In one embodiment, step (S22) is bypassed, step (S2) comprises sub-stepsof:

-   -   step (T21), providing a carbon nanotube layer 122 comprising a        plurality of carbon nanotubes; and    -   step (T22), forming a HfO₂ layer 124 on surfaces of the        plurality of carbon nanotubes to form a carbon nanotube/HfO₂        composite layer.

Step (S21) further comprises a step of laying the carbon nanotube layer122 on a mounting plate, the mounting plate can be a glass, a metalframe, or at least two supports arranged at a certain distance. When thecarbon nanotube layer 122 is laid on the mounting plate, the method forforming the at least one functional layer 120 further comprisesseparating the carbon nanotube/HfO₂ composite layer from the mountingplate. In one embodiment, the carbon nanotube layer is laid on the metalframe, and the carbon nanotube/HfO₂ composite layer is separated fromthe metal frame by laser cutting.

The carbon nanotube layer 122 comprises one carbon nanotube film or atleast two carbon nanotube films stacked and crossed with each other. Inone embodiment, the carbon nanotube film is drawn from a carbon nanotubearray via a stretch tool. The carbon nanotube film is directly laid onthe separator substrate 110 after being drawn from the carbon nanotubearray. In one embodiment, a height of the carbon nanotube array is about300 micrometers. A diameter of the carbon nanotubes of the carbonnanotube array can range from about 20 nanometers to about 30nanometers. A method of the carbon nanotube film being drawn is taughtby U.S. Pat. No. 8,048,256 to Feng et al. In one embodiment, the carbonnanotube layer 122 comprises 5 carbon nanotube films stacked andvertically crossed with each other, step (S21) comprises steps of:laying a first carbon nanotube film on a surface of the metal frame;laying a second carbon nanotube film on a surface of the first carbonnanotube film, wherein a first extending direction of the carbonnanotubes in the first carbon nanotube film is substantiallyperpendicular with a second extending direction of the carbon nanotubesin the second carbon nanotube film. The above steps are repeated untilthe carbon nanotube layer comprising 5 carbon nanotube films stacked andvertically crossed with each other is obtained.

In step (S22), in one embodiment, reactive ions are used to etch thecarbon nanotube layer 122. The reactive ions etching can be implementedby using oxygen plasma, argon plasma, or the like. In one embodiment,the reactive ions etching is implemented by using the oxygen plasma inan etching device. A flow rate of the oxygen plasma is about 30 sccm toabout 50 sccm and a pressure is about 5 Pa to about 15 Pa. A power isabout 15 W to about 25 W, and an etching time is about 5 seconds toabout 15 seconds. In one embodiment, the reactive ions etching isimplemented by using the oxygen plasma, the flow rate of the oxygenplasma is about 40 sccm, the pressure is about 10 Pa, the power is about20 W, and the etching time is about 10 seconds. When the reactive ionsetching is implemented by using the oxygen plasma, physical defects andoxygen-containing functional groups can be simultaneously formed on thesurface of each carbon nanotube. The oxygen-containing functional groupsare beneficial for a surface adsorption of hafnium source and anadsorption of polar polysulfides, the functional groups can also improveelectrolyte wettability of intermediate functional layers.

In step (S23), in one embodiment, the HfO₂ layer 124 is continuous onsurfaces of the plurality of carbon nanotubes. The HfO₂ layer 124 can beformed on surfaces of the plurality of carbon nanotubes by an atomiclayer deposition (ALD) method, which comprises:

-   -   step (S231), putting the carbon nanotube layer into a deposition        chamber;    -   step (S232), applying a hafnium precursor into the deposition        chamber, and vacuum pumping after a first reaction period;    -   step (S233) forming a cycle of hafnium oxide deposition layer on        surfaces of the plurality of carbon nanotubes by applying an        oxygen precursor into the deposition chamber and vacuum pumping        after a second reaction period; and    -   step (S234), repeating step (S232) and step (S233) at least one        time to form the hafnium oxide layer.

In step (S232), in one embodiment, the hafnium precursor is hafniumtetra chloride (HfCl₄) gas. The first reaction period is about 0.3seconds to about 0.7 seconds. A first vacuum pumping time is about 1second to about 3 seconds.

In step (S233), in one embodiment, the oxygen precursor is water vapor(H₂O). The second reaction period is about 0.1 seconds to about 0.4seconds. A second vacuum pumping time is about 0.5 seconds to about 2seconds.

The hafnium precursor and the oxygen precursor can be applied in thedeposition chamber by carrier gas. The carrier gas can comprises highpurity nitrogen or high purity argon. A flow rate of the carrier gas isabout 150 sccm to about 200 sccm.

In step (S234), in one embodiment, the step (S232) and step (S233) arerepeated 17 to 21 times to obtain 18 to 22 cycles of HfO₂ depositionlayers. A thickness of each HfO₂ deposition layer is about 0.1nanometers to about 0.2 nanometers.

In one embodiment, the carrier gas is the high purity nitrogen, and theflow rate of the carrier gas is about 200 sccm. The method for formingthe continuous hafnium oxide layer 124 in this embodiment comprisesapplying the HfCl₄ gas into the deposition chamber through the highpurity nitrogen, exposing the carbon nanotube layer 122 to the HfCl₄ gasfor 0.5 seconds, and vacuum pumping for 2 seconds. The water vapor isbrought into the deposition chamber through the high purity nitrogen, toobtain one cycle of HfO₂ deposition layer after 0.25 seconds reactiontime, wherein the thickness of the cycle of HfO₂ deposition layer isabout 0.1 nanometers to about 0.2 nanometers. The above steps arerepeated 21 times.

Referring to FIG. 9, one embodiment is described in relation to alithium-sulfur battery 20. The lithium-sulfur battery 20 comprises apositive electrode 201, a negative electrode 202, a lithium-sulfurbattery separator 10, and an electrolytic solution 203. Thelithium-sulfur battery separator 10 is located between the positiveelectrode 201 and the negative electrode 202. The positive electrode 201is a composite electrode comprising sulfur and carbon nanotubes, and aweight ratio between the sulfur and the positive electrode is about 65wt % to about 70 wt %. An area density of the sulfur is about 1.80mg/cm² to about 2.10 mg/cm². The carbon nanotubes have high strength asa mechanical property, conductivity, and large aspect ratio, thus thepositive electrode 201 has excellent mechanical properties andconductivity even if the positive electrode is free of polymer binder.The lithium-sulfur battery also has large energy density even if thelithium-sulfur battery is free of polymer binder. The negative electrodeis a metallic lithium foil. The lithium-sulfur battery separator is thelithium-sulfur battery separator 10.

A method for making the composite electrode comprising sulfur and carbonnanotubes comprises making oxidized carbon nanotubes, dispersing theoxidized carbon nanotubes and a sulfur powder in a solution byultrasound agitation, and vacuum filtering and drying to obtain asulfur-carbon nanotube composite film. The sulfur-carbon nanotubecomposite film is heat treated to obtain the composite electrodecomprising sulfur and carbon nanotubes.

In one embodiment, the method for making the composite electrodecomprising sulfur and carbon nanotubes comprises dispersing a carbonnanotube array in a mixed solution of HNO₃ and H₂SO₄, wherein a massratio of the HNO₃ and the H₂SO₄ is about 3:1, and heating the mixedsolution of HNO₃ and H₂SO₄ to 80° C. This temperature is maintained for4 hours to obtain the oxidized carbon nanotubes. The oxidized carbonnanotubes and the sulfur powder are dispersed in a mixed solution ofethanol and water by ultrasonically treating for 30 minutes at 1000 Wpower. Vacuum filtering and drying at 50° C. are applied to obtain thesulfur-carbon nanotube composite film and the sulfur-carbon nanotubecomposite film is placed in a stainless steel reactor and heat treatedat 155° C. for 8 hours.

EXAMPLE 1

In the lithium-sulfur battery of this example, the positive electrode isthe composite electrode comprising sulfur and carbon nanotubes. Thenegative electrode is the metallic lithium foil. The electrolyticsolution is 1 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI)solution with 0.2 M LiNO₃ as additive.

The lithium-sulfur battery separator comprises the separator substrateand ten functional layers located on one surface of the separatorsubstrate, the ten functional layers are stacked with each other. Theseparator substrate is a polypropylene film. Each of the ten functionallayers comprises a carbon nanotube layer and a HfO₂ layer. The carbonnanotube layer comprises two drawn carbon nanotube films stacked andvertically crossed with each other. The thickness of the HfO₂ layer isabout 2.0 nanometers to about 3.0 nanometers.

COMPARATIVE EXAMPLE 1

In this comparative example, the lithium-sulfur battery is the same asthat in Example 1, except that the lithium-sulfur battery separator is apolyethylene film. The polyethylene film is the same as that in Example1.

Referring to FIG. 10, the lithium-sulfur battery in Example 1 is chargedand discharged at a constant rate of 1.0 C, after a charge/dischargecycle is performed 500 times. A capacity of the lithium-sulfur batteryof Example 1 is about 721 mA h g⁻¹, a capacity retention ratio of thelithium-sulfur battery of Example 1 is about 67.8%. However, a capacityof the lithium-sulfur battery of Comparative Example 1 is only 600 mA hg⁻¹ after the charge/discharge cycle is performed 150 times, and thelithium-sulfur battery of Comparative Example 1 is short-circuitedduring the 280th charge/discharge cycle. It shows that the capacity andthe capacity retention ratio of the lithium-sulfur battery of Example 1are greatly improved compared with the lithium-sulfur battery ofComparative Example 1.

Referring to FIG. 11, it can be seen that the lithium-sulfur battery ofExample 1 has two typical discharge plateaus of 2.35V and 2.10V, whereinthe high voltage plateau 2.35 V is for a conversion from cyclo-S₈ topolysulfides, and the low voltage plateau 2.10V is for a major dischargeprocess from polysulfides to Li₂S₂/Li₂S. It shows that the loss ofactive sulfur in the lithium-sulfur battery is greatly suppressed by thelithium-sulfur battery separator of Example 1, the shuttle effect isavoided between the cathode and the anode, and the electrochemicalreactivity of the lithium-sulfur battery is improved. It can also beseen that the electrode exhibits a stable voltage plateau as the currentdensity increases, and electrode polarization is small at differentcurrent densities. The electrochemical kinetics are improved.

Referring to FIG. 12, it can be seen that a main voltage plateau of thelithium-sulfur battery of Comparative Example 1 degrades severely as thecurrent density increased to 2 C, which shows that most polysulfides haddissolved into the electrolyte and a further nucleation conversion toLi₂S₂/Li₂S was not performed at high rates.

Referring to FIG. 13, it can be seen that the lithium-sulfur battery ofExample 1 demonstrates discharge capacities of 1255 mA h g⁻¹, 1107 mA hg⁻¹, 1014 mA h g⁻¹, 970 mA h g⁻¹, 918 mA h g⁻¹, and 800 mA h g⁻¹ at 0.2C, 0.5 C, 1 C, 5 C, 7 C, and 10 C, respectively. Compared with thelithium-sulfur battery of Comparative Example 1, the lithium-sulfurbattery of Example 1 has larger discharge capacity and smaller batterycapacity decay ratio.

Referring to FIG. 14, it can be seen that after 100 charge/dischargecycles at constant rate of 0.2 C, a discharge capacity of thelithium-sulfur battery of Example 1 is about 947 mA h g⁻¹, and acoulombic efficiency is up to 88% . However, the lithium-sulfur batteryof Comparative Example 1 exhibited an obvious capacity fading process,after 100 charge/discharge cycles at constant rate of 0.2 C, a dischargecapacity of the lithium-sulfur battery of Comparative Example 1 is about582 mA h g⁻¹, and a coulombic efficiency is only up to 50%.

Referring to FIG. 15, the lithium-sulfur battery of Example 1 still hasexcellent stability after standing for 20 days. After 20charge/discharge cycles at constant rate of 0.2 C, a discharge capacityretention ratio of the lithium-sulfur battery of Example 1 is about90.8%, and the lithium-sulfur battery of Example 1 shows excellentstability during 100 charge/discharge cycles process. However, thedischarge capacity of the lithium-sulfur battery of Comparative Example1 has an obvious decay after standing for 20 days, after 20charge/discharge cycles at constant rate of 0.2 C, a discharge capacityretention ratio of the lithium-sulfur battery of Comparative Example 1is only 64.5%. The lithium-sulfur battery in Example 1 was substantiallyfree from self-discharge, however, the lithium-sulfur battery inComparative Example 1 shows serious self-discharge phenomenon.

It is to be understood that the above-described embodiments are intendedto illustrate rather than limit the present disclosure. Variations maybe made to the embodiments without departing from the spirit of thepresent disclosure as claimed. Elements associated with any of the aboveembodiments are envisioned to be associated with any other embodiments.The above-described embodiments illustrate the scope of the presentdisclosure but do not restrict the scope of the present disclosure.

Depending on the embodiment, certain of the steps of a method describedmay be removed, others may be added, and the sequence of steps may bealtered. The description and the claims drawn to a method may includesome indication in reference to certain steps. However, the indicationused is only to be viewed for identification purposes and not as asuggestion as to an order for the steps.

What is claimed is:
 1. A method for making a lithium-sulfur batteryseparator comprising: step (S1), providing a separator substratecomprising a first surface and a second surface opposite to the firstsurface; and step (S2), forming a functional layer on the first surface,wherein forming the functional layer on the first surface comprises:step (S21), forming a carbon nanotube layer comprising a plurality ofcarbon nanotubes on the first surface; step (S22) forming defects onsurfaces of the plurality of carbon nanotubes by etching the carbonnanotube layer; and step (S23) applying a hafnium oxide layer on thedefects to form a carbon nanotube/hafnium oxide composite layer.
 2. Themethod of claim 1, wherein etching the carbon nanotube layer isperformed by oxygen plasma in an etching device.
 3. The method of claim2, wherein a flow rate of the oxygen plasma is about 30 sccm to about 50sccm, a pressure of the etching device is about 5 Pa to about 15 Pa, apower of the etching device is about 15 W to about 25 W, and an etchingtime is about 5 seconds to about 15 seconds.
 4. The method of claim 1,wherein the hafnium oxide layer is formed on surfaces of the pluralityof carbon nanotubes by an atomic layer deposition method, and the atomiclayer deposition method comprises: step (S231), putting the carbonnanotube layer into a deposition chamber; step (S232), applying ahafnium precursor into the deposition chamber, and vacuum pumping aftera first reaction period; step (S233) forming a cycle of hafnium oxidedeposition layer on surfaces of the plurality of carbon nanotubes byapplying an oxygen precursor into the deposition chamber and vacuumpumping after a second reaction period; and step (S234), repeating step(S232) and step (S233) at least one time to form the hafnium oxidelayer.
 5. The method of claim 4, wherein the hafnium precursor ishafnium tetra chloride gas.
 6. The method of claim 5, wherein the firstreaction period is about 0.3 seconds to about 0.7 seconds; and a firstvacuum pumping time is about 1 second to about 3 seconds.
 7. The methodof claim 4, wherein the oxygen precursor is water vapor.
 8. The methodof claim 7, wherein the second reaction period is about 0.1 seconds toabout 0.4 seconds; and a second vacuum pumping time is about 0.5 secondsto about 2 seconds.
 9. The method of claim 4, wherein the hafniumprecursor and the oxygen precursor are applied in the deposition chamberby a carrier gas, and a flow rate of the carrier gas is about 150 sccmto about 200 sccm.
 10. The method of claim 4, wherein the step (S232)and step (S233) are repeated 17 to 21 times, and the hafnium oxide layercomprises 18 to 22 cycles of hafnium oxide deposition layers.
 11. Themethod of claim 4, wherein a thickness of each cycle of hafnium oxidedeposition layer is about 0.1 nanometers to about 0.2 nanometers. 12.The method of claim 1, wherein step (S2) further comprises forming atleast two carbon nanotube/hafnium oxide composite layers on the firstsurface.
 13. The method of claim 1, wherein: step (S21) furthercomprises laying the carbon nanotube layer on a mounting plate; step(S2) further comprises a step (S24), comprising separating the carbonnanotube/hafnium oxide composite layer from the mounting plate afterstep (S23).
 14. The method of claim 1, further comprising forminganother functional layer on a second surface opposite to the firstsurface.
 15. The method of claim 1, wherein the carbon nanotube layercomprises one carbon nanotube film or at least two carbon nanotube filmsstacked and crossed with each other.
 16. The method of claim 15, whereinthe carbon nanotube film is drawn from a carbon nanotube array anddirectly laid on the separator substrate after being drawn from thecarbon nanotube array.
 17. The method of claim 1, wherein the hafniumoxide layer is a continuous film.
 18. A method for making alithium-sulfur battery separator comprising: step (T1), providing aseparator substrate comprising a first surface and a second surfaceopposite to the first surface; and step (T2), forming a functional layeron the first surface, wherein forming the functional layer on the firstsurface comprises: step (T21), forming a carbon nanotube layercomprising a plurality of carbon nanotubes on the first surface; andstep (T22), forming a hafnium oxide layer on surfaces of the pluralityof carbon nanotubes to form a carbon nanotube/hafnium oxide compositelayer.
 19. The method of claim 18, wherein in step (S2), forming atleast two carbon nanotube/hafnium oxide composite layer s stacked on thefirst surface.
 20. The method of claim 18, wherein the carbon nanotubelayer comprises one carbon nanotube film or at least two carbon nanotubefilms stacked and crossed with each other, the carbon nanotube film isdrawn from a carbon nanotube array and directly laid on the separatorsubstrate after being drawn from the carbon nanotube array.